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A reactive leukemoid pattern can also be left shifted. It more often shows neutrophil predominance with toxic granulation, Döhle bodies, or cytoplasmic vacuolation. Those reactive toxic changes argue toward inflammation or infection, but morphology alone cannot exclude a clonal process. Leukocyte alkaline phosphatase is historical context and is not modern confirmation.

CML is defined by BCR::ABL1, usually associated with t(9;22)(q34;q11), the Philadelphia chromosome. Cytogenetics, fluorescence in situ hybridization (FISH), and reverse-transcription polymerase chain reaction (RT-PCR) answer related but different questions. A cryptic rearrangement is a BCR::ABL1 rearrangement not visible by conventional chromosome banding. FISH can detect a rearrangement but does not identify the transcript or additional chromosomal abnormalities.

At diagnosis, bone marrow morphology and conventional chromosome banding are integrated with BCR::ABL1 testing. RT-PCR can use peripheral blood or marrow and identifies the transcript needed for follow-up. Common p210 transcripts are e13a2 and e14a2, historically b2a2 and b3a2. Report the detected transcript in current nomenclature and state the method limitation when common-transcript coverage cannot exclude an atypical fusion.

Comparison diagram contrasts a broad granulocytic maturation spectrum with basophilia in a CML-supportive pattern against neutrophil predominance with toxic changes in a reactive leukemoid pattern. — Figure 1CML-supportive and reactive leukemoid patterns compared as evidence, not diagnostic labels.

Knowledge checks

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Knowledge check 1

Which findings more often support a reactive leukemoid pattern?

Choose at least 2 options.

Toxic granulation

Döhle bodies or vacuolation

Absolute basophilia with a broad maturation spectrum

Leukocyte alkaline phosphatase as modern confirmation

Knowledge check 2

Which statement best describes the complementary methods in initial CML evaluation?

Choose one option.

Integrate morphology, BCR::ABL1 testing, and chromosome analysis because each method has limits

Use FISH alone because it identifies transcript and additional abnormalities

Use conventional cytogenetics alone because it detects every rearrangement

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